This invention relates to a continuous laser beam welding process for galvanized steel sheet members and, more particularly, to such galvanized steel laser welding process carried out in the presence of an oxygen rich cover gas to reduce zinc vapor porosity within the weld nugget.
In a common laser welding process, metal members are assembled with bonding surfaces in juxtaposition, for example, to form a lap joint, and one outer surface is scanned with a continuous laser beam to melt and fuse the members at the bonding surfaces. In contrast to other welding processes such as electrical resistance welding that generate heat concentrated at the bonding surfaces, laser welding heats a zone extending from the irradiated surface down below the touching surfaces to create a pool of molten metal within both members and that, upon solidification, forms the weld nugget that joins the two sheet members together. In order to minimize oxidation of the molten surface, it has heretofore been common practice to protect the irradiated surface using an inert cover gas such as helium.
In general, galvanized steel sheet comprises a low carbon steel core sheet coated with a metallic zinc plate to enhance corrosion protection. We have found that a problem is created during laser welding of two side-galvanized steel sheets positioned for a lap joint or the like. We have seen this problem whether we weld under an inert gas atmosphere or in air. The problem is the weld track is porous, brittle and weak.
We think that this problem arises because of vaporization of the zinc coating at the pressed-together bonding surfaces at temperatures below the temperature required to fuse the steel. At the outer surface which receives the incident laser beam, the zinc vapor is simply expelled into the atmosphere by the energy of the beam and does not affect the quality of the weld. However, the zinc vapor generated by the heated coating at the submerged lapped bonding surfaces is entrapped within the molten steel. The gas tries to flow through the molten metal and expel molten metal from the weld track, which generates pores within the weld nugget and weakens the joint.
We have discovered that such laser welding of galvanized steel members in the presence of an oxygen-enriched cover gas significantly reduces zinc vapor porosity and irregular cuts in the weld nugget. This is attributed to reaction between the oxygen and the zinc vapor that produces solid zinc oxide that becomes harmlessly dispersed throughout the weld metal. It is also attributed to the surface tension of the molten pool being favorably affected by the presence of oxygen, which helps to minimize vapor expulsion. The formation of a weld nugget that is substantially pore free using oxygen cover gas is in dramatic contrast to the porous nuggets formed using inert cover gas or even air.
Therefore, it is an object of this invention to provide an improved laser welding process for joining lapped steel sheet members, at least one of which comprises a buried galvanized bonding surface, to produce a strong, substantially pore-free weld nugget.